A series of chemical and enzymatic processes are known or have been suggested for the quantitative determination of cholesterol in various materials and especially in biological fluids. In the case of these processes, it is almost always necessary to introduce into the measurement cholesterol solutions with definite, known content of cholesterol as standard for the evaluation of the measurement results. For this purpose, use is made of cholesterol solutions with a defined cholesterol content, which are called cholesterol standards.
For the determination of cholesterol in biological fluids, as a rule standard solutions are necessary, the properties of which substantially coincide with those of the biological fluid. In general, this requirement can be fulfilled by the use of aqueous cholesterol standard solutions. In the case of a cholesterol standard solution, it is also important that it be sufficiently stable for comparatively long periods of time and that the cholesterol content remain absolutely constant. Hitherto, a series of aqueous cholesterol standard solutions have become known with a stable cholesterol content which remains constant for a comparatively long period of time. Thus, for example, Federal Republic of Germany patent specification No. 23 24 386 describes and claims an aqueous cholesterol standard solution which, in addition to a solubiliser, for example oxypolyethoxydodecane, and other component materials, contains from 1 to 20 volume percent of a primary or secondary aliphatic alcohol containing up to 4 carbon atoms. This cholesterol standard solution has proved to be stable and, even after storage for several years under normal conditions, shows an unchanged cholesterol content.
Federal Republic of Germany patent specification No. 28 39 433 describes an aqueous lipid standard solution which, in addition to a lipid, contains an ionic and a non-ionic detergent. With this composition there is said to be achieved, in particular, that, in the case of unavoidable standing of the sample, it evaporates as little as possible and the viscosity of the standard solution is not influenced.
Finally, from Clin. Chem., 25, 132-135/1979, there is known an aqueous cholesterol standard solution which, in addition to a definite and constant amount of cholesterol, contains sodium desoxycholate as solubiliser and which has proved to be useful for the enzymatic determination of cholesterol.
The previously known cholesterol standard solutions are admittedly more or less useful for enzymatic cholesterol determinations by the so-called end value methods but they are useless for a kinetic cholesterol determination, such as is described, for example, in Federal Republic of Germany patent specification No. 30 46 241.0. For this purpose, it is necessary to use a cholesterol standard which, in addition to the previously demanded requirements for such a standard with regard to stability and viscosity, must fulfill the additional requirement that its kinetic behaviour be identical to that of the sample to be determined. The previously known cholesterol standard solutions are useless for a kinetic cholesterol determination since their kinetic behaviour, because of their detergent content, differs considerably from that of the cholesterol samples to be measured.